A selective call communication system comprises a transmitter and a number of receivers. Each receiver or pager is designed to intercept the same carrier wave, but its audio circuitry is rendered operative only when the carrier wave is modulated by a predetermined code. Upon reception of such code, the receiver produces an audible or visual alerting signal. In certain kinds of systems, the signal is followed immediately by a voice message. In others, no voice message is transmitted. The possessor of the latter type, upon being alerted, will perform some previously agreed upon action such as calling his office.
Usually a selectively called receiver will automatically turn off or become "squelched" after a communication to it has been completed. It will become reactivated only upon reception of that same code. In the interim, other receivers are being called in sequence and the communications with such receivers are not being heard by the first-mentioned receiver.
There are situations when the receiver having been once enabled by the correct code should remain indefinitely enabled to reproduce all communications on the channel. In other words, it should become carrier squelched and reproduce all communications on the carrier wave corresponding to that receiver, irrespective of whether they are preceded by the proper code. For other users of the system, and perhaps for the same user at other times, the receivers should remain code responsive.
In the past, there have been receivers which will respond to a predetermined code and then automatically become carrier squelched to reproduce all modulation on the channel. Such a receiver is disclosed in U.S. Pat. No. 3,613,004 which issued to one of the applicants named herein. If the carrier wave to the receiver described therein is interrupted it will revert to its code-squelch mode.
Motorola's MINITOR alert monitor has a switch that allows the user to select between a carrier-squelch mode and a tone-squelch mode. When the switch is in the position corresponding to the latter, the proper code will activate the receiver and also cause it to change to the carrier-squelch mode, in which mode all transmissions on the channel will be heard. The receiver will be squelched or quieted between transmissions. A separate reset bar is actuated to cause the alert monitor to revert to the tone-squelch mode. The alert monitor can be placed in its carrier-squelch mode manually by placing the select switch in its other position. Thus, the MINITOR alert monitor requires two separate manual switches to place the receiver into, and take it out of, the tone-squelch and carrier-squelch modes. Furthermore, the MINITOR alert monitor has no means to become unsquelched indefinitely so that the receiver output is continuously on.
In Motorola's PAGECOM radio pager, reception of the proper code unsquelches the receiver completely so that it is on all of the time; i.e., it becomes a monitor. The pager may then be reset at any time so that subsequent communications are detected only if preceded by the proper code. However, the PAGECOM pager has no means to enable manual placement in this mode.
Neither the MINITOR alert monitor nor the PAGECOM radio paper are programmable in one way so that the button can be used to place the receiver into and take it out of the carrier-squelch mode, or programmable in another way so that the button can be used to place the receiver into, and take it out of, the monitor mode.
Communication receivers of the kind described in this application are portable and therefore incorporate batteries as the source of power. The batteries may be rechargeable (such as nickel-cadmium) or throw away (such as mercury). Rechargeable batteries are much more expensive, larger and perhaps heavier than their counterpart throw-away batteries of equivalent capacity. A rechargeable battery must be replaced from time to time since it eventually will wear out. Also, rechargeable batteries must be frequently taken out of service in order to charge them. On the other hand, throw-away batteries must be replaced often, and, therefore, their use is likely to be more expensive. In order to use throw-away batteries, it has been proposed to increase their useful lives by reducing the power consumed by the receiver. Battery saver circuits in the past have accomplished this by delivering pulsed power to the various circuits in the receiver until a message for that receiver is received, at which time the power becomes continuous.
Carrier-operated battery saver circuits cause power to be delivered to the receiver continuously whenever the carrier wave is present. When the receiver is part of a system on a busy channel the carrier wave is present much of the time and battery life is, therefore, not prolonged very much. In receivers with code-operated battery saver circuits, power is continuously applied only when a code for that particular receiver is received. Such a receiver will consume substantial power only when its code is being received and for the duration of the ensuing voice message. Some additional power will be consumed by the receiver while it is receiving a proper first tone even if the code is not meant for it. One such battery saver circuit is disclosed in U.S. Pat. No. Re. 28,022, the patentee of which is one of the applicants herein.
In tone-operated battery saver circuits, the pulses of power are spaced apart as much as possible in order to minimize power consumption. However, in order to be sure the pager will respond to its code consisting of a sequence of tones, the first tone must have a duration longer than the time between pulses. When such a receiver is converted to one that is carrier squelched after having received a proper code, as described above, it is undesirable for the pulses of power to be so far apart. The initial part of an ensuing voice communication would be lost to the extent it fell between such pulses. If the time between pulses was say 1.75 seconds to increase battery life, then up to 1.75 seconds of conversation could be lost because the receiver would not have continuous power at such time.
A further consideration in the design of battery saver circuits is the duration of the pulses. Since it is during those pulses that the receiver is consuming power, it is desirable to minimize their durations. However, those pulses must be long enough to insure that when the first tone of a possibly correct code is received, it will be detected and an output produced to cause the battery saver to provide continuous power for at least a time. In the past, detection of the first tone has taken too long so that the duration of the pulses produced by battery saver circuits were unduly long.
In certain selective call communication systems, a sequence of sinusoidal tones at predetermined frequencies is modulated onto the carrier wave. In each receiver, the modulation components are separated from the carrier wave and a sequence of sinusoidal tones is provided. The tones are normally limited; that is, the tones are converted into a square wave. The square wave is applied to a set of filters, or a sequentially tunable filter, which provides a sinusoidal output if the input tones have the frequencies and are in the order to which the particular receiver is designed to respond. In the past, the sine wave was applied to a detector which included an RC integrating circuit and provided an output when its amplitude exceeded some threshold. U.S. Pat. No. 3,613,004 cited above discloses such a detector. The time between inception of the tone and detection thereof varied depending on precise values of the resistance and capacitance in the detector. Also, the extent of precision of the tuning of the filter, the particular tone frequency, supply voltage variations, and noise content in the signal, all affected the time between tone inception and detection. The tones used in systems with receivers having such a detector must be long enough to take care of the longest time lag which might be encountered.
The processor circuit, which includes the RF input circuit, local oscillators, IF stages and the demodulator, produces noise spikes and other glitches which may include frequency components on the frequency of the decoder filters so as to be coupled therethrough. Delay circuitry prevents the spike or glitch from being reflected in circuitry down the line. Each tone, on the other hand, has a duration greater than the delay, so that it will be reflected in subsequent circuits. In the past, such delay circuits injected more delay than needed, in order to compensate for tolerances in the decoder components and for the effect of variations in temperature and supply voltage. Since the noise spikes and other glitches commonly are on the order of twenty microseconds or less in duration, more delay was unnecessary. In receivers with battery saver circuits, it is important as explained previously, that detection of the first tone occur quickly in order that the battery saver pulses can be as narrow as possible. The delay caused by these prior-art delay circuits requires longer battery saver pulses.
Inititally, there were two basic kinds of pagers available: tone-only pagers which emitted an alerting tone when paged and tone-and-voice pagers which emitted an alerting tone when paged, followed by a voice message. In the case of a tone-only pager, the alerting tone would tell the person wearing the pager to perform some previously agreed upon action. Later, pagers were developed which could perform in a tone-only mode or in a tone-and-voice mode depending on the character of the code applied thereto. U.S. Pat. No. 4,019,142, in which the patentee is one of the present applicants, discloses such a pager.
Also, it became clear that a single alerting tone did not convey enough information. For example, in certain instances a caller may wish to tell the wearer of a pager to call his office, and in other instances the caller may wish to tell him to call his home. Pagers were developed to furnish different alerting tones in accordance with the nature of the code applied thereto.
In Motorola's METRO-PAGEBOY radio pager, for example, an interrupted alert tone is emitted when a preamble and five tones are received by the pager. A continuous alert tone is emitted when the preamble and six tones are received. This necessitates an additional code tone to create the different alerting tone.
An alternative scheme is disclosed in U.S. Pat. No. 3,686,635 to Millington et al. In the pager described in this patent, a standard two-tone sequence causes an interrupted alerting tone to be generated, and when it is desired to make a group call, a longer single tone is sent which causes the pager to generate a continuous alerting tone. The Millington et al. pager does not have means to provide a selectively called pager with one of a plurality of alerting tones or to be converted to a tone-and-voice pager.
U.S. Pat. No. 4,019,142 discloses a receiver which responds to different durations of a code tone to display numbers corresponding to the code or to switch automatically between a tone-only mode and a tone-and-voice mode. The receiver disclosed therein does not provide different alerting tones in accordance with the durations of the tones in the code itself.
In this kind of receiver, a tone in the code can have one of several durations. Therefore, it is important that the receiver be capable of accurately and consistently distinguishing one from the other. Otherwise, a code tone of given duration which is supposed to cause an alerting tone of certain characteristics will instead cause production of a different alerting tone. This aspect is of particular concern if the possible durations are not very different.
The circuitry in U.S. Pat. No. 4,019,142 which detects the tone duration is RC in nature and, therefore, is not capable of distinguishing, with high accuracy, between different, but closely related, durations.
A pager has as many channels as there are tones in the code. The first channel is normally operative while the rest are normally inoperative. When the first tone is received, the first channel develops an output to render the second channel operative so that it can process the second tone. The second channel then develops an output to render the third channel operative so that it can process the third tone, and so forth. It has been learned that there are several advantages to be gained by rendering operative a normally inoperative channel only on termination of the preceding tone. For example, the second channel would become operative only after the first tone has terminated, not during the first tone. Such operation is disclosed in U.S. Pat. No. 2,929,921 to Clark, Jr. and also in the above-mentioned '004 patent. In each case, an RC circuit in the output of the first channel produces an enabling signal to turn on the second channel for a predetermined time.
It is desirable that the "window" (the time during which the second channel is operative) be as short as possible in order to minimize the possibility of a particular pager responding to another tone which appears to correspond to its second tone, but is, in fact, part of speech or the code for some other receiver. The window must be long enough so that the second channel can detect the second tone. The timing circuits disclosed in these two patents being RC in nature must compensate for tolerances in the components and for temperature and supply voltage variations. The window was therefore longer than needed.
Generally, resistors and capacitors in timing circuits render it impractical to integrate them. In other words, usable capacitive elements cannot readily be formed in an IC. Instead, capacitors must be connected externally to ICs in order to provide adequate timing capability.
Circuits which time digitally offer the very substantial advantage of being integrateable. Certain timing functions in selective call communication receivers have in the past been performed digitally. For example, in Motorola's METRO-PAGEBOY pager, digital timing is employed. However, the METRO-PAGEBOY pager does not use digital timing for delay, envelope detecting, duration detecting, timing of the "window" for enablement of a second channel, and other timing functions.
Selective call communication receivers which are responsive to two or more tones are often responsive to a long first tone in order to enable communication with a group of them. However, such receivers are not readily modifiable to a single-tone receiver, it being desirable, in certain instances, to use a single-tone system comprising many single-tone pagers.